This invention relates to modular electric drive systems and, more particularly, to electric drive systems which provide scalable power levels.
Conventional electric drive systems for supplying power to power conversion equipment that will be utilized to drive a propulsion motor for a ship or provide auxiliary power in the ship, for example, require large transformers, generally 12 pulse transformers. Such conventional electric drive systems usually require a different transformer design for each of several scalable power levels, resulting in disadvantages for the respective size weight and packaging arrangements that cannot be optimized based on the use of a single large transformer.
Medium voltage static AC drives benefit from transformers that can improve power quality and provide isolation. In the case of pulse width modulated (PWM) low voltage drives, several modules are placed in series to obtain higher output voltages and in parallel to provide a higher output currents, providing multi-level power converters. These drives require an isolated input for each module, usually from multiple isolated transformer secondaries, often phase shifted, to provide a required power output. For large motor drives, the transformer volume and weight can become excessive due to the core size, the secondary connections and the cooling methods used. Transformers with high output power ratings and multiple secondary connections are very large and cumbersome. When multiple secondaries with phase shifted outputs are required, a single large core is mechanically impracticable in some applications due to the complexity of the secondary configuration.
The Uhlmann U.S. Pat. No. 2,820,189 describes a static current converter using voltage commutation. As described in that patent, four six-pulse, two-way connected converters are connected to two transformers so as to form a 24-pulse converter with a phase angle of 15xc2x0. The Uhlmann patent discloses an arrangement of voltage commutating static current converters operating in different phase so that a short-circuiting of voltage harmonics through the constituent converters is prevented.
The Hammond U.S. Pat. No. 5,625,545 and the Opel et al. U.S. Pat. No. 5,638,263 disclose power conversion systems which utilize phase shifting of a three-phase AC input to provide multiple phases to reduce output ripple and, in this case, H-bridges are provided to effect the switching of different phase components. These patents disclose multiple, isolated secondaries in the input transformer, and delta, wye, extended delta, or zig-zag configurations which have an effect on phase shift. The Opel et al. Patent also discloses the use of multiple transformers rather than a single transformer with one primary winding and multiple secondary windings and different arrangements of power converters based on current and voltage output requirements.
The Maezawa et al. U.S. Pat. No. 5,671,127 describes a high voltage, large output DC power supply device which is of a relatively small size and capable of reducing harmonic currents in its DC output energy. This is accomplished by increasing the number of phases using separate transformers which are connected to provide twelve output phases with primary windings connected in delta and star configurations for shifting the output phases.
Such prior art power supply or conversion arrangements utilize phase shifting transformers to shift from a three phase AC input to provide multiple phase outputs in order to reduce output ripple. None of the prior art transformer arrangements is capable of satisfying the transformer requirements of different power supply arrangements with a minimum weight and volume for each application.
Accordingly it is an object of the present invention to provide a modular transformer arrangement for multi-level power converters which overcomes disadvantages of the prior art.
Another object of the invention is to provide a power dense modular three dimensional arrangement of low power transformers for use in high power systems so as to avoid the need for multiple transformers of varying design to satisfy varying power requirements for different equipment.
A further object of the invention is to provide a transformer arrangement which eliminates the need for a single large transformer for power electric drive systems which is sometime difficult to manufacture, install and replace.
These and objects of the invention are attained by providing a modular transformer arrangement containing transformer modules which can be used in single or multiple units connected in series and/or in parallel with phase shifting to provide a high power output of improved quality. By utilizing a transformer arrangement made of modular transformer units with the ability to add or subtract units, the transformer output is scaleable for different applications so as to assure the minimum possible transformer size and weight for each requirement. In addition, the manufacture, installation and repair of transformer arrangements are facilitated by the use of modular units.
In a preferred embodiment of the invention, a modular transformer arrangement is connected to supply power to power conversion equipment for driving a motor at variable speed. In other embodiments, a modular transformer arrangement supplies power to power conversion equipment for AC or DC service supply or UPS equipment. Preferably, a modular transformer arrangement combines series and parallel modules in a manner that produces a drive that is scaleable in power with respect to both voltage and current and in phases, utilizing a single common power transformer modular design. For power conversion a high power design is provided that utilizes multiple small modular transformers that produce isolated phase shifted sources of power to an inverter.